1. Field of Invention
This invention relates to communication systems. Specifically, the present invention relates to systems for predicting the signal to interference and noise ratio (SINR) of a received signal to facilitate data rate control in wireless communication systems.
2. Description of the Related Art
Wireless communication systems are used in a variety of demanding applications including search and rescue and business applications. In addition, wireless communication systems are increasingly employed to transfer computer data in office network and Internet applications. Such applications require efficient and reliable communication systems that can effectively operate in electrically fading and noisy environments and that can handle high data transfer rates.
Cellular telecommunication systems are characterized by a plurality of mobile stations (e.g. cellular telephones or wireless phones) in communication with one or more base stations. The communications link from a base station to a mobile station is the forward link. The communications link from the mobile station to the base station is the reverse link.
Signals transmitted by a mobile station are received by a base station and often relayed to a mobile switching center (MSC). The MSC in turn routes the signal to a public switched telephone network (PSTN) or to another mobile station. Similarly, signals are often transmitted from the public switched telephone network to a mobile station via a base station and a mobile switching center. Each base station governs a cell, a region within which a mobile station may communicate via the base station.
In typical mobile communication systems, information is encoded, modulated, and transmitted over a channel and received, demodulated and decoded by a receiver. In many modern communication systems, such as Code Division Multiple Access (CDMA) cellular networks, the information is encoded digitally for channel noise, capacity, and data security reasons. A convolutional encoder or turbo encoder often performs the encoding of the information.
As is well known in the art, a convolutional encoder converts a sequence of input data bits to a codeword based on a convolution of the input sequence with itself or with another signal. Code rate and generating polynomials are used to define a convolutional code. Convolutional encoding of data combined with a Viterbi decoder is a well-known technique for providing error correction coding and decoding of data. Turbo encoders employ turbo codes, which are serial or parallel concatenations of two or more constituent codes such as convolutional codes.
Mobile communication systems are typified by the movement of a receiver relative to a transmitter or vice versa. The communications link between transmitters and receivers in a mobile communication system is a fading channel. Mobile satellite communications systems, having a transmitter on a spacecraft and a receiver on a ground based vehicle, cellular telephone systems and terrestrial microwave systems are examples of fading communication systems. A fading channel is a channel that is severely degraded. The degradation results from numerous effects including multipath fading, severe attenuation due to the receipt via multiple paths of reflections of the transmitted signal off objects and structures in the atmosphere and on the surface, and from interference caused by other users of the communications system. Other effects contributing to the impairment of the faded channel include Doppler shift due to the movement of the receiver relative to the transmitter and additive noise.
Typically, an information signal is first converted into a form suitable for efficient transmission over the channel. Conversion or modulation of the information signal involves varying a parameter of a carrier wave on the basis of the information signal in such a way that the spectrum of the resulting modulated carrier is confined within the channel bandwidth. At a user location, the original message signal is replicated from a version of the modulated carrier received subsequent to propagation over the channel. Such replication is generally achieved by using an inverse of the modulation process employed by the source transmitter.
In a CDMA system, all frequency resources are allocated simultaneously to all users of the cellular network. Each user employs a noise-like wide band signal occupying the entire frequency allocation. The encoder facilitates the encoding of necessary redundant data within each transmission frame to take advantage of the entire frequency allocation, and also facilitates the variable rate transmission on a frame by frame basis.
For voice communication, the capacity of a CDMA system is maximized by having each user transmit only as much data as is necessary. This is because each user""s transmission contributes incrementally to the interference in a CDMA communication system. A very effective means of reducing each user""s burden on capacity without reducing the quality of service to that user is by means of variable rate transmission. The use of a variable rate communication channel reduces mutual interference by eliminating unnecessary transmissions when there is no useful speech to be transmitted.
Due to the characteristics of voice communication, power control is typically utilized in a CDMA system to guarantee each user a reliable link for certain fixed data rates. Vocoders can provide variable rate source coding of speech data, using the technique described in U.S. Pat. No. 5,414,796, May 9, 1995, entitled xe2x80x9cVariable Rate Vocoderxe2x80x9d. Once a vocoder generates a sequence of information bits at certain rate, power control will try to adjust the user to transmit as little power as possible that can reliably support the rate. Power control, thus by suppressing each user""s contribution to the total interference, facilitates the maximum capacity of a CDMA voice system in the sense that the number of active users is maximized.
For data communication, the parameters, which measure the quality and effectiveness of a system, are the transmission delay required for transferring a data packet and the average throughput rate of the system. Transmission delay is an important metric for measuring the quality of the data communication system. The average throughput rate is a measurement of the efficiency of the data transmission capacity of the communication system. In order to optimize the above parameters for a data communication system, rate control, instead of power control, is typically utilized. The above differences between the voice and data communication systems can be better understood by the following different characteristics between the voice and data communications.
A significant difference between voice services and data services is the fact that the former imposes stringent and fixed delay requirements. Typically, the overall one-way delay of speech frames must be less than 100 msec. In contrast, the data delay can become a variable parameter used to optimize the efficiency of the data communication system. Specifically, more efficient error correcting coding techniques that require significantly larger delays than those that can be tolerated by voice services can be utilized. An exemplary efficient coding scheme for data is disclosed in U.S. patent application Ser. No. 5,933,462, entitled xe2x80x9cSOFT DECISION OUTPUT DECODER FOR DECODING CONVOLUTIONALLY ENCODED CODEWORDSxe2x80x9d, filed Nov. 6, 1996, assigned to the assignee of the present invention and incorporated by reference herein.
Another significant difference between voice services and data services is that the former requires a fixed and common grade of service (GOS) for all users. Typically, for digital systems providing voice services, this translates into a fixed and equal transmission rate for all users and a maximum tolerable value for the error rates of the speech frames. In contrast, for data services, the GOS can be different from user to user and can be a parameter optimized to increase the overall efficiency of the data communication system. The GOS of a data communication system is typically defined as the total delay incurred in the transfer of a predetermined amount of data, hereinafter referred to as a data packet.
Yet another significant difference between voice services and data services is that the former requires a reliable communication link which, in the exemplary CDMA communication system, is provided by soft handoff. Soft handoff results in redundant transmissions from two or more base stations to improve reliability. However, this additional reliability is not required for data transmission because the data packets received in error can be retransmitted. For data services, the transmit power used to support soft handoff can be more efficiently used for transmitting additional data. A method and apparatus which is optimized for the wireless transmission of digital data is described in U.S. patent application Ser. No. 08/963,386 entitled xe2x80x9cMethod and Apparatus For Higher Rate Packet Data Transmissionxe2x80x9d, which is assigned to the assignee of the present invention and incorporated by reference herein.
As a conclusion of the above characteristics of data communication, a data communication system designed to optimize the average throughput will attempt to serve each user from the best serving base station and at the highest data rate Rb which the user can reliably support. The above conclusion is disclosed in U.S. patent application Ser. No. 08/963,386 entitled xe2x80x9cMethod and Apparatus For Higher Rate Packet Data Transmissionxe2x80x9d, which is assigned to the assignee of the present invention and incorporated by reference herein. As a result of the above conclusion, in the modern high-data-rate (HDR) system, a base station always transmits at maximum power to only one user at each time slot and uses rate control to adjust the maximum rate that the user can reliably receive. As a characteristic of data communication, throughput is more important to the forward link than reverse link.
A proper rate control algorithm contains 2 loops, an inner loop and an outer loop. The inner loop controls the forward-link data rate based on the difference between the average SINR of the next packet and the SINR thresholds of all the data rates, while the outer loop adjusts the SINR thresholds of the data rates based on the forward link PER. For convenience, the average SINR of a packet and the SINR thresholds of all data rates will be referred to as packet SINR and SINR thresholds, respectively.
The SINR thresholds reflect the performance of the modem design, but are mainly determined by the channel statistics. We expect that the SINR thresholds change slowly with relatively small variances, thus a tracking loop based on PER will achieve good performance. Further details and analysis on how the outer loop can be done is out of the scope of this study.
In this patent, we assume that the SINR thresholds are fixed. We will focus on the design of the inner loop algorithm. The core technique inside the inner loop is channel prediction.
In an HDR system, forward-link traffic channels support 11 data rates, each data rate corresponding to a deterministic packet length associated with 1, 2, 4, 8 or 16 slots. Some packet lengths can support multiple rates. Typically, higher rates are associated with shorter packet lengths.
The predictor will predict the next packet SINR for all packet lengths. The mobile will attempt to request the highest rate by comparing the predictions with the SINR thresholds. For convenience, the prediction of the next packet SINR for a given packet length will be simply referred to as prediction.
In the HDR system, the data rate request information is sent to the BS over the reverse-link data rate control (DRC) channel once every slot. The BS includes a scheduler that schedules forward link traffic packets in accordance with a fair and efficient priority algorithm. Once the scheduler decides to serve a mobile, the mobile is served at the rate it requested over the DRC channel (the actual rate may be lower if the BS does not have enough information bits).
Upon receipt of the data rate request message, the base station adjusts the rate of a transmitted signal. The adjustments are performed for the next packet in response to information provided about the channel by a previous packet. A base station broadcasting at insufficient or excessive data rates results in reduced channel throughput or inefficient use of network resources, respectively.
Current implementations of the above technique however, have significant limitations. The SINR may change rapidly. The data rate that was appropriate for a previously transmitted packet may not be appropriate for a subsequently transmitted packet. The delay between the transmission of one packet and the generation and transmission of a data rate request message for a subsequent packet can result in reduced channel throughput, especially when the channel is characterized by rapid fluctuations in noise or other interference.
Hence, a need exists in the art for an efficient system and method for maximizing communication systems throughput that accounts for a changing SINR occurring between the determination of the rate control signal based on a previous packet and the application of the rate control signal to a subsequent packet. There is a further need for a system for adjusting the data rate of a transmitted signal in accordance with the changing SINR.
The need in the art is addressed by the system for providing an accurate prediction of a signal-to-interference noise ratio of the present invention. In the illustrative embodiment, the inventive system is employed in a wireless communications system and includes a first mechanism for receiving a signal transmitted across a channel via an external transmitter. A second mechanism generates a sequence of estimates of signal-to-interference noise ratio based on the received signal. A third mechanism determines a relationship between elements of the sequence of estimates. A fourth mechanism employs the relationship to provide a signal-to-interference noise ratio prediction for a subsequently received signal.
In the illustrative embodiment, the inventive system further includes a mechanism for generating a data rate request message based on the signal-to-noise ratio prediction. A transmitter transmits the data rate request message to the external transceiver. The external transceiver includes rate control circuitry for receiving the data rate request message and adjusting a transmission rate of the signal in response thereto.
In the specific embodiment, the relationship between elements of the sequence of estimates is based on an average of the elements of the sequence of estimates. The third mechanism includes a bank of filters for computing the average. The impulse responses of the transfer functions associated with each filter in the bank of filters are tailored for different fading environments. The different fading environments include one environment associated with a rapidly moving system, a second environment associated with a slowly moving system, and a third system associated with a system moving at a medium velocity.
A selection mechanism is connected to each of the filter banks and selects an output from one of the filters in the filter bank. The selected output is associated with a filter having a transfer function most suitable to a current fading environment. In the present specific embodiment, the largest output is selected from the outputs of the filter bank based on the smallest error standard deviation. The resulting accurate prediction of the signal-to-interference noise ratio facilitates generating accurate rate requests.